As an example of an image forming apparatus that forms an image electrophotographically, JP-A-2007-178657 describes a color laser printer having a drum unit that is removably mounted to a body casing. The drum unit has a plurality of drum subunits provided in accordance with respective colors, and a pair of side plates with the drum subunits sandwiched therebetween. Each drum subunit has a photosensitive drum, and the photosensitive drum is positioned using the side plates.
Each side plate has a positioning shaft portion and a cutout portion. In addition, the body casing has a press arm and a reference shaft. In a state where the drum unit is mounted in the body casing, the positioning shaft portion is pressed against the press arm, and the cutout portion is in contact with the reference shaft. Therefore, the drum unit is positioned in the body casing.